The perinuclear theca is a condensed cytosolic protein layer that surrounds the mammalian sperm nucleus except in the region of tail implantation. It is resistant to non-ionic detergent extraction and makes up most of the cytosol of the sperm head. The perinuclear theca can be divided both structurally and compositionally into subacrosomal and postacrosomal regions. Most of the sequence-identities of investigated perinuclear theca proteins have been unexpected, revealing novel proteins as well as isoforms of somatic and conventionally nuclear proteins. Their characterizations have led us to propose that the perinuclear theca can be regionalized into two functional categories: the subacrosomal layer, involved in acrosomal assembly, and the postacrosomal sheath, involved in sperm-egg interactions during fertilization such as egg activation. Immunocytochemical investigation of the origins of well-characterized perinuclear theca proteins have led us to propose that: the subacrosomal layer is assembled relatively early in spermiogenesis from cytosolic proteins that "piggy-back" their way to the forming perinuclear theca on the periphery of acrosomic vesicles, while the postacrosomal sheath is assembled later, from cytosolic proteins that are transported up the microtubular manchette as it descends over the caudal half of the elongating spermatid nucleus. In this review data collected on resident perinuclear theca proteins, SubH2Bv, RAB2, PAWP and the four core somatic histones, is used to substantiate these hypotheses.